In internal combustion engines, the bearing assemblies typically each comprise a pair of half bearings retaining a crankshaft that is rotatable about an axis. Each half bearing is a hollow generally semi-cylindrical bearing shell, and typically at least one is a flange half bearing, in which the bearing shell is provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In other bearing assemblies it is also known to use an annular or circular thrust washer.
The bearing surfaces of sliding bearings generally have a layered construction, in which a strong backing material is coated with one or more layers having preferred tribological properties, which provide the bearing surface that faces the cooperating moving part, in use. In known half bearings and thrust washers: the strong backing material may be steel, having a thickness of about 1 mm or more; the bearings surface is coated with a layer (the “overlay layer”) of 6 to 25 μm of a plastic polymer-based composite layer; and optionally at least one intervening layer (a “lining layer”) may be provided between the backing and the overlay layer, such as a copper-based material (e.g. copper-tin bronze) or an aluminium-based material (e.g. aluminium-tin alloy), adhered to the backing, and the thickness of the optional lining layer is generally in the range from about 0.1 to 0.5 mm (e.g. 300 am of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities).
For example, WO2010066396 describes a plastic polymer-based composite material for use as an overlay layer on a steel backing, which comprises a matrix of a polyamide/imide plastic polymer material, having distributed throughout the matrix: from 5 to less than 15% vol of a metal powder; from 1 to 15% vol of a fluoropolymer particulate, the balance being the polyamide/imide resin apart from incidental impurities (e.g. a layer of 12 am thickness that comprises 12.5% vol Al, 5.7% vol PTFE particulate, 4.8% vol silane, <0.1% vol other components, and balance (approximately 77% vol) polyamide/imide).
Such plastic polymer-based overlay layers may be deposited by various different methods, including spraying, pad printing (an indirect offset printing process, e.g. in which a silicone pad transfers a patterned layer of the plastic polymer composite material onto the sliding bearing substrate), screen printing, or by a transfer rolling process. Prior to deposition, the plastic polymer is in solution in a solvent, and the solid particulate is suspended in the solution. The composite overlay layer may be built up as a succession of sub-layers, in which repeated depositions of thin coatings are interspersed with flash-off phases to remove the solvent. After the deposition has been completed, the deposited polymer is thermally cured by prolonged heating to set the polymer by inducing cross-linking, e.g. at 140 to 240° C. for a duration that may range from a few minutes to a few hours (e.g. 10 minutes to 2 hours).
The polymer may be cured at different temperatures, when deposited on different substrates. For example: for polymer deposited directly onto a steel backing without a lining layer, or where a bronze lining layer is provided on the backing, the substrate is able to withstand high temperature curing of the polymer; in contrast, where an aluminium-tin lining layer is used on the backing, lower temperature curing is used, to avoid migration of the tin to crystal boundaries, weakening the lining layer. The polymer layer typically obscures the backing or any lining layer, and does not significantly change in appearance when cured, or differ in appearance when cured under different conditions. Accordingly, to distinguish between differently cured bearings formed with the same uncured plastic polymer-based layer, it is necessary to refer to the documentation on which the curing regime has recorded.